8,842 research outputs found
A comparison of the responses of mature and young clonal tea to drought.
To assist commercial producers with optimising the use of irrigation water, the
responses to drought of mature and young tea crops (22 and 5 years after field
planting respectively) were compared using data from two adjacent long-term
irrigation experiments in Southern Tanzania. Providing the maximum potential
soil water deficit was below about 400-500 mm for mature, and 200-250 mm for
young plants (clone 6/8), annual yields of dry tea from rainfed or partially
irrigated crops were similar to those from the corresponding well-watered crops.
At deficits greater than this, annual yields declined rapidly in young tea (up
to 22 kg (ha mm)-1) but relatively slowly in mature tea (up to 6.5 kg (ha mm)-
1). This apparent insensitivity of the mature crop to drought was due
principally to compensation that occurred during the rains for yield lost in the
dry season. Differences in dry matter distribution and shoot to root ratios
contributed to these contrasting responses. Thus, the total above ground dry
mass of well-irrigated, mature plants was about twice that for young plants.
Similarly, the total mass of structural roots (>1 mm diameter), to 3 m depth,
was four times greater in the mature crop than in the young crop and, for fine
roots (<1 mm diameter), eight times greater. The corresponding shoot to root
ratios (dry mass) were about 1:1 and 2:1 respectively. In addition, each unit
area of leaf in the canopy of a mature plant had six times more fine roots (by
weight) available to extract and supply water than did a young plant. Despite
the logistical benefits resulting from more even crop distribution during the
year when crops are fully irrigated, producers currently prefer to save water
and energy costs by allowing a substantial soil water deficit to develop prior
to the start of the rains, up to 250 mm in mature tea, knowing that yield
compensation will occur later
Stability criterion for self-similar solutions with a scalar field and those with a stiff fluid in general relativity
A stability criterion is derived in general relativity for self-similar
solutions with a scalar field and those with a stiff fluid, which is a perfect
fluid with the equation of state . A wide class of self-similar
solutions turn out to be unstable against kink mode perturbation. According to
the criterion, the Evans-Coleman stiff-fluid solution is unstable and cannot be
a critical solution for the spherical collapse of a stiff fluid if we allow
sufficiently small discontinuity in the density gradient field in the initial
data sets. The self-similar scalar-field solution, which was recently found
numerically by Brady {\it et al.} (2002 {\it Class. Quantum. Grav.} {\bf 19}
6359), is also unstable. Both the flat Friedmann universe with a scalar field
and that with a stiff fluid suffer from kink instability at the particle
horizon scale.Comment: 15 pages, accepted for publication in Classical and Quantum Gravity,
typos correcte
An overview of tea research in Tanzania - with special reference to the Southern Highlands.
The history of tea development in Tanzania from the early part of this century
to the present is summarised. Average yields of made tea from well managed
estates in the Mufindi district have increased from around 600 kg ha-1 in the
late 1950s to 3000 kg ha-1 at the present time: by comparison, yields from
smallholder farms have remained much lower, averaging only 400-500 kg ha-1.
There have been a large number of technical, economic and other changes over the
last 30 to 40 years. The removal of shade trees, the use of herbicides, the
application of NPK compound fertilisers, the introduction of irrigation (on some
estates) and changes in harvesting policy have all contributed to the increases
in yield. Financial and infrastructural problems have contributed to the low
yields from many smallholders and others, and have limited the uptake of new
technology. The contribution of research is reviewed, from the start of the Tea
Research Institute of East Africa in Kenya in 1951, through to the development
of the Marikitanda Tea Research Centre in Amani in 1967; the Ngwazi Tea Research
Unit in Mufindi (1967 to 1970, and from 1986), and lastly the Kifyulilo Tea
Research Station, also in Mufindi in 1986. The yield potential of well
fertilized and irrigated clonal tea, grown at an altitude of 1800 m, is around
6000 kg ha-1. This potential is reduced by drought, lack of fertilizer, bush
vacancies and inefficient harvesting practices. The corresponding potential
yields at high (2200 m) and low (1200 m) altitude sites range from 3000-3500 kg
ha-1 up to 9000-10000 kg ha-1 and are largely a function of temperature. The
opportunities for increasing yields of existing tea, smallholder and estate, are
enormous. Tea production in the Southern Highlands of Tanzania is about to
expand rapidly. Good, appropriate research is needed to sustain this development
over the long term, and suggestions on how best this is done in order to assist
the large scale producers as well as the smallholders, are discussed
Signatures of superconducting gap inhomogeneities in optical properties
Scanning tunneling spectroscopy applied to the high- cuprates has
revealed significant spatial inhomogeneity on the nanoscale. Regions on the
order of a coherence length in size show variations of the magnitude of the
superconducting gap of order or more. An important unresolved question
is whether or not these variations are also present in the bulk, and how they
influence superconducting properties. As many theories and data analyses for
high- superconductivity assume spatial homogeneity of the gap magnitude,
this is a pressing question. We consider the far-infrared optical conductivity
and evaluate, within an effective medium approximation, what signatures of
spatial variations in gap magnitude are present in various optical quantities.
In addition to the case of d-wave superconductivity, relevant to the high-
cuprates, we have also considered s-wave gap symmetry in order to provide
expected signatures of inhomogeneities for superconductors in general. While
signatures of gap inhomogeneities can be strongly manifested in s-wave
superconductors, we find that the far-infrared optical conductivity in d-wave
is robust against such inhomogeneity.Comment: 8 pages, 7 figure
Multiple imaging by gravitational waves
Gravitational waves act like lenses for the light propagating through them.
This phenomenon is described using the vector formalism employed for ordinary
gravitational lenses, which was proved to be applicable also to a
non-stationary spacetime, with the appropriate modifications. In order to have
multiple imaging, an approximate condition analogous to that for ordinary
gravitational lenses must be satisfied. Certain astrophysical sources of
gravitational waves satisfy this condition, while the gravitational wave
background, on average, does not. Multiple imaging by gravitational waves is,
in principle, possible, but the probability of observing such a phenomenon is
extremely low.Comment: 23 pages, LaTeX, no figures, to appear in Int. J. Mod. Phys.
Magnetic cloaking by a paramagnet/superconductor cylindrical tube in the critical state
Cloaking of static magnetic fields by a finite thickness type-II
superconductor tube being in the full critical state and surrounded by a
coaxial paramagnet shell is studied. On the basis of exact solutions to the
Maxwell equations, it is shown that, additionally to previous studies assuming
the Meissner state of the superconductor constituent, perfect cloaking is still
realizable at fields higher than the field of full flux penetration into the
superconductor and for arbitrary geometrical parameters of both constituents.
It is also proven that simultaneously the structure is fully undetectable under
the cloaking conditions. Differently from the case of the Meissner state the
cloaking properties in the application relevant critical state are realized,
however, only at a certain field magnitude.Comment: 5 pages, 4 figures; to be published in Applied Physics Letters. arXiv
admin note: substantial text overlap with arXiv:1401.356
Discrete soliton mobility in two-dimensional waveguide arrays with saturable nonlinearity
We address the issue of mobility of localized modes in two-dimensional
nonlinear Schr\"odinger lattices with saturable nonlinearity. This describes
e.g. discrete spatial solitons in a tight-binding approximation of
two-dimensional optical waveguide arrays made from photorefractive crystals. We
discuss numerically obtained exact stationary solutions and their stability,
focussing on three different solution families with peaks at one, two, and four
neighboring sites, respectively. When varying the power, there is a repeated
exchange of stability between these three solutions, with symmetry-broken
families of connecting intermediate stationary solutions appearing at the
bifurcation points. When the nonlinearity parameter is not too large, we
observe good mobility, and a well defined Peierls-Nabarro barrier measuring the
minimum energy necessary for rendering a stable stationary solution mobile.Comment: 19 pages, 4 figure
A complete classification of spherically symmetric perfect fluid similarity solutions
We classify all spherically symmetric perfect fluid solutions of Einstein's
equations with equation of state p/mu=a which are self-similar in the sense
that all dimensionless variables depend only upon z=r/t. For a given value of
a, such solutions are described by two parameters and they can be classified in
terms of their behaviour at large and small distances from the origin; this
usually corresponds to large and small values of z but (due to a coordinate
anomaly) it may also correspond to finite z. We base our analysis on the
demonstration that all similarity solutions must be asymptotic to solutions
which depend on either powers of z or powers of lnz. We show that there are
only three similarity solutions which have an exact power-law dependence on z:
the flat Friedmann solution, a static solution and a Kantowski-Sachs solution
(although the latter is probably only physical for a1/5, there are
also two families of solutions which are asymptotically (but not exactly)
Minkowski: the first is asymptotically Minkowski as z tends to infinity and is
described by one parameter; the second is asymptotically Minkowski at a finite
value of z and is described by two parameters. A complete analysis of the dust
solutions is given, since these can be written down explicitly and elucidate
the link between the z>0 and z<0 solutions. Solutions with pressure are then
discussed in detail; these share many of the characteristics of the dust
solutions but they also exhibit new features.Comment: 63 pages. To appear in Physical Review
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